The present invention relates in general to power management systems and subsystems, and is particularly directed to a new and improved power management scheme for (customer premises installed) digital data services (DDS) telecommunication equipment, that is configured to interface the DDS equipment with both a local utility power source and a telecommunication link (span) that is remotely powered by way of a telecommunication facility, such as a central office. The inventive power management scheme includes an auxiliary energy storage element, such as a capacitor, that stores energy from the local utility power source and the span. The invention monitors the power availability for and requirements of the DDS equipment; if the local utility power is insufficient to maintain proper operation of the DDS equipment, a limited quantity of power is drawn from the span.
In addition to sending voice and/or data over the telephone lines, a telecommunication service provider may use the same wires to power circuits, such as DDS terminal devices that are connected to the signal lines at remote locations. The power that is used to operate telecommunication equipment via the signal lines is sourced from the telephone company""s central office (CO). This method of remote line powering is commonly referred to as xe2x80x98loopxe2x80x99 or xe2x80x98spanxe2x80x99 powering, and permits the phone companies to remotely power terminal equipment that is up to several miles away from the central office. Loop powering is customarily used extensively on mid-span devices (rather than termination devices) in DDS, ISDN, T1, HDSL, and other digital networks. The device at the remote end of the link may or may not continuously need this span-supplied power, or may use this power only during emergency situations. For purposes of the present description, the equipment at the remote end of the link will be referred to as terminating equipment, and typically resides at a customer""s premises.
Within the telephone networks of the continental United States, terminating devices for digital data services are generally not span-powered. Instead, the terminating device is typically located within the customer""s premises and receives its power from the customer""s local utility feed. In normal operating conditions, the only current drawn by the xe2x80x98locally poweredxe2x80x99 termination device from the network signal wires is what is commonly termed xe2x80x98sealingxe2x80x99 or xe2x80x98wettingxe2x80x99 current. Wetting current is not necessary for the equipment to operate, but is used to reduce corrosion of the wires and splices between the phone company and the remote location.
However, when local utility power is lost at the customer""s premises (for example during a lighting storm), the terminating device will stop functioning. This is problematic for digital services (such as ISDN) that implement plain-old-telephone-service (POTS) support. Should a local power failure occur, there will be a loss of phone service (including emergency service, such as the ability to place a 911 call). As a consequence, customers have traditionally been required to maintain a separate standard analog phone line in addition to the ISDN digital data line(s).
There is currently interest in the both U.S. and abroad to configure terminating devices that can be span-powered. Although there is no applicable U.S. regulation or tariff for span-powering customer premises equipment, the European Telecommunications Standards Institute (ETSI) has released an international technical report ETR080, as a precursor to a possible specification. This technical report recommends that the central office supply between 51 VDC and 115 VDC on the span (depending on its length). The effect of line resistance requires that power drawn by terminal equipment be less than a prescribed value (1100 mW) during worst case conditions. This constitutes a potential problem in the U.S., since a standard ringer equivalency number (REN) load of five (North American) consumes more instantaneous power than is available from the span.
In accordance with the present invention, these concerns are successfully addressed by a dual power management system, that is readily integrated with customer premises-installed digital data services terminal equipment, and is operative to controllably power such equipment from either or both of a local power source and the span. The invention automatically switches between local powering and remote powering as necessary for terminal equipment operation and sinks wetting current from the network during local powering.
As will be described, the invention is configured to monitor the power availability for and power requirements of termination equipment and, in response to local power being insufficient to maintain operation, derives from the phone line up to a maximum amount of power, which is limited to a fixed value that prevents the line from collapsing, and satisfies current industry proposals, such as that contained in the above-referenced ETR080 report. The inventive power management scheme includes an auxiliary energy store that is controllably operative to store a limited quantity of energy from the local utility interface or the span, so as to enable extended power operation (including ringing the phone).
The general architecture of the dual power management system of the present invention includes a local utility interface and a span interface. The local utility interface has barrierxe2x80x94isolation transformer circuitry configured to connect to a local utility interfacexe2x80x94typically, an AC wall outletxe2x80x94which isolates and transforms the utility voltage to a safe level, such as 42 VDC. A main power converter is coupled to receive the local utility voltage and supplies various regulated voltages as required for terminal equipment communication subsystems.
In addition to being coupled to the local utility interface, the main power converter is coupled to a power limited converter within the span interface. Normally, the span interface provides a signaling interface with the telephone lines, and sinks wetting current. In addition, in response to insufficient local power, the power limited converter derives power from the span to maintain terminal equipment operation. Controlled extraction of a limited amount of power from the span is carried out by a power-limited converter, in response to a reduction or loss of power from the local utility. During this xe2x80x98spanxe2x80x99xe2x80x94powered mode, the power-limited converter controllably charges an auxiliary energy storage capacitor that is coupled in circuit with each of the local utility interface section, the span interface section and the main power converter. The auxiliary energy storage capacitor is normally charged by transformed voltage from the local utility interface.
A power availability sensing and control unit is coupled to a terminal device control processor and to associated interface circuits, and serves to monitor a number of operational parameters, including performance of the local utility interface, the span, and the amount of energy stored in the auxiliary storage capacitor. This information is used to control various power management subsystems, such as modulation of the span interface section""s power limited converter, current limit, start-up control, undervoltage lockout, etc. Undervoltage lockout (UVLO) and start-up circuits are included to ensure that the power circuits are disabled for conditions outside their intended operating range, and to provide for a smooth start-up when power is initially applied.
Should there be a failure or unacceptable reduction of the local utility power, the sensing and control unit will switch the operation of the power management system to xe2x80x98span-poweredxe2x80x99 mode, so that the power limited converter within the span interface unit will begin drawing the maximum power available from the network, until the auxiliary energy storage capacitor is fully charged. Once the auxiliary storage capacitor is fully charged, the span power will be modulated so as to draw only energy necessary for the terminal equipment to operate.
Should the terminal equipment require more than the limited amount of power allowed to be drawn from the span (such as for ringing the phone), the power limited converter will again pull this maximum power from the network; any deficit in required energy will be drawn from the auxiliary storage capacitor. Once the capacitor has been fully recharged, the power limited converter will again reduce the power transfer. In the case of a severely excessive ringing load, the storage capacitor will not have sufficient energy to power the terminal equipment for the full cadence period of the ringing signal. If the capacitor voltage decreases to near the minimum value necessary for the equipment to operate, the sensing and control unit will temporarily inhibit ringing, in order to prevent the terminal equipment from shutting down. Ringing will then continue during the next cadence period.
The power limited converter circuitry includes an electronic switch installed in circuit with the primary winding of a transformer, which has a secondary winding coupled in a charge path to the auxiliary energy storage capacitor. Conduction through the switch is modulated by adjusting the frequency of an output waveform produced by a variable frequency oscillator. When a peak detector senses that the value of the primary current has reached a prescribed level, it signals a control circuit to turn the electronic switch off, so as to interrupt the current flow through the primary winding, causing energy stored in the transformer to be transferred to the capacitor as the secondary current decays. The decay of the secondary current occurs at a rate determined by the capacitor voltage and secondary inductance. After the energy transfer and a certain amount of idle time has elapsed, this cycle repeats, so that a power-limiting function is achieved by controllably modulating the charging of the capacitor.
When the terminal equipment is being powered by the local utility interface, the control processor is operative to reduce the frequency of the output waveform generated by the variable frequency oscillator to its minimum value, so that the power being delivered by the span is reduced to only the level of wetting current. In the absence of or insufficient local power, however, the oscillator frequency is set to either a maximumxe2x80x94providing a maximum amount of limited powerxe2x80x94or modulated between its minimum and maximum values for energy storage regulation.